1. Field of the Invention
The present invention relates to a process for producing microcapsules for electrophoretic display devices, as well as microcapsules for electrophoretic display devices obtained by this process, and their applications.
2. Description of the Related Art
An electrophoretic display device displays character data, image data, and the like, by the behavior of electrophoretic particles, for example, when a voltage is applied to a dispersion in which the electrophoretic particles are dispersed in a solvent. For example, if the electrophoretic particles and the solvent are colored with different colors, the color of the electrophoretic particles is observed when the electrophoretic particles are moved to the surface of the solvent by voltage application, and the color of the dispersion is observed when the electrophoretic particles are moved to the bottom of the solvent. If electrodes to which voltage application is made possible by address appointment are provided, different colors for the respective addresses can be displayed, and accordingly, arbitrary character data and image data can be displayed. Moreover, it is possible to rewrite the display data, and at the same time, it is advantageous that the display data can be maintained as it is, even if there is no electric signal.
In recent years, in place of the conventional electrophoretic display devices (e.g., see Japanese Patent Publication (Kokoku) No. Sho 50-15115) in which a dispersion of electrophoretic particles are enclosed in spaces between opposed electrode substrates, there have been developed microcapsule type electrophoretic display devices (e.g., see Japanese Patent No. 2551783) with a structure in which microcapsules enclosing a dispersion of electrophoretic particles are arranged between opposed electrode substrates. As compared with the conventional electrophoretic display devices, the microcapsule type electrophoretic display devices are remarkably improved in various performances and functions, such as long-term stability of display, responsiveness, contrast, and display rewritable times.
For the electrophoretic display devices, there have also been proposed, besides fixed equipments such as ordinary displays, techniques of flexible displays, which have the shape of a thin sheet, which are ultraportable, and which can freely be inflected, such as electronic papers and electronic books.
In such electrophoretic display devices, there has been proposed a technique of making a dispersion of electrophoretic particles, i.e., a dispersion for electrophoretic display devices, into microcapsules (see, e.g., Japanese Patent Laid-open Publication (Kohyo) No. 2002-526812). A flexible sheet-shaped electrophoretic display device can easily be produced by enclosing a dispersion for electrophoretic display devices in microsphere-shaped microcapsules made of a transparent resin or the like and allowing the thus obtained microcapsules for electrophoretic display devices to be supported on the surface of an electrode film. There occurs neither uneven local distribution nor movement of a dispersion for electrophoretic display devices by making the dispersion for electrophoretic display devices into microcapsules, resulting in a technique also suitable for various applications in which electronic equipments may variously change their positions or may be inflected when used, such as electronic papers.
The sheet-shaped electrophoretic display device having flexibility is produced by, for example, by applying a coating solution for electrophoretic display devices, which contains microcapsules for electrophoretic display devices and a binder resin, to a conductive layer of an electrode film to produce a sheet for electrophoretic display devices, having a data display layer on the conductive layer of the electrode film; and then attaching another electrode film to the data display layer. At that time, the microcapsules are arranged on the electrode film without leaving voids in the in-plane directions and without being layered in the thickness direction, so that high reflectance of white display, lower reflectance of black display, and consequently higher contrast can be obtained.
Thus, microcapsules for electrophoretic display devices are desired to have particle diameters as uniform as possible. However, microcapsules for electrophoretic display devices, which are produced by conventional techniques, show a certain broad particle size distribution of particle diameter. That is, there were able to be obtained only microcapsules for electrophoretic display devices, having a great coefficient of variation (CV value) of particle diameter. Therefore, in order to obtain microcapsules for electrophoretic display devices, having a small coefficient of variation (CV value) of particle diameter, for example, classification treatment using a sieve or a slit has been carried out (e.g., see Japanese Patent Laid-open Publication (Kohyo) No. 2002-526812).
However, in the classification treatment using a sieve or a slit, for example, relatively soft microcapsules such as microcapsules for electrophoretic display devices, each enclosing a dispersion containing electrophoretic particles and a solvent in a shell made of gelatin and gum arabic, may be deformed to become thin and long and may easily slip through the sieve or the slit, even if the microcapsules have particle diameters equal to or greater than the meshes of the sieve or the aperture of the slit, and therefore, it was not able to carry out classification with high precision. Further, since the microcapsules become condensed on the sieve or the slit, the microcapsules having smaller particle diameters may adhere to the microcapsules having greater particle diameters, and therefore, it was also not able to carry out classification with high precision. Moreover, since the microcapsules are rubbed on the sieve or the slit by vibration or pressure for a long time, the microcapsules may suffer from great physical damage, which has became a cause of lowering in their strength.
As methods other than classification treatment using a sieve or a slit, there may be mentioned a spontaneous settling method making use of gravity and a wet classification method making use of centrifugal force; however, the former requires a long time for treatment and is inferior both in productivity and yield, and the latter has a problem that microcapsules may be broken by a pump to be used for solution sending in the continuous treatment, and therefore, both methods are not suitable for classification on an industrial scale.